1. Field of the Invention
The present invention relates to a transistor module incorporating semiconductor devices and the like, and more particularly to a technique of reducing a transient voltage which is generated owing to an external lead terminal thereof.
2. Description of the Conventional Art
In the design and manufacture of electronic equipment, in terms of a reduction in the number of circuit component parts used, as compared with an arrangement in which its electronic circuits are formed by electronic components such as transistors, it is more advantageous to use a transistor module in which a plurality of electronic components are formed as a module in an outer-packaging resin or metal case by being mounted on a circuit board. In such a transistor module, as shown in FIG. 6, an external lead terminal 22 is led from the interior of a transistor module 21 to the exterior of an outer-packaging resin (not shown). This external lead terminal 22 has an intermediate terminal portion 25 bent in the form of, for instance, the letter U and disposed between its internal terminal portion 23 and external terminal portion 24. As a result, even if the transistor module 21 produces heat during the operation and the external lead wire 22 undergoes heat deformation, a bent portion 25a of the intermediate terminal portion 25 absorbs this heat deformation and alleviates a stress with respect to the encapsulating resin, thereby preventing a decline in the reliability of the transistor module 21. In addition, as water or the like is prevented from entering the interior of the transistor module 21 along the external lead terminal 22, its environmental resistance is improved.
However, if the intermediate terminal portion 25 having the bent portion 25a is provided in the external lead terminal 22 of the transistor module 21, a large inductance L.sub.0 parasitically occurs in correspondence with the bent shape. For this reason, if the supply of a current with respect to the transistor module 21 is switched on and off at high speed, when the current decreases sharply, a large transient voltage .DELTA.V.sub.0 expressed by -L.sub.0 (dI.sub.0 /dt) occurs in correspondence with a change in its current dI.sub.0 /dt. Such a transient voltage .DELTA.V.sub.0 can cause a malfunction or the like in the circuit. Here, to reduce the .DELTA.V.sub.0, it suffices if the intermediate terminal portion 25 is formed into a linear structure to reduce the inductance L.sub.0. This, however, impairs the effect of alleviating the heat deformation of the external lead terminal 22 and the effect of enhancing the environmental resistance, thereby causing a decline in the reliability of the transistor module 21. For this reason, with the conventional transistor module 21, there has been the problem that either the reliability or the transient voltage .DELTA.V.sub.0 must be sacrificed.
Accordingly, as shown in FIG. 7, it is conceivable to use an external lead terminal 32 in which a slit 35a is provided in an intermediate terminal portion 35 in the longitudinal direction thereof to provide a plurality of parallel current paths between an external terminal portion 34 and an internal terminal portion 33. In the external lead terminal 32 having such a structure, since the current path is divided, even if the current is switched at high speed, a current I.sub.1 flowing through each current path is small, so that a transient voltage .DELTA.V.sub.1 expressed by -L.sub.1 (dI.sub.1 /dt) is small. However, in the case where the transient voltage .DELTA.V.sub.1 is reduced by dividing the current path by providing the slit 35a, the effect of reducing the transient voltage .DELTA.V.sub.1 is restricted by the number of divisions of the current path. Hence, to reduce the .DELTA.V.sub.1 sufficiently, it is necessary to form a multiplicity of grooves. Accordingly, there are restrictions arising from the sizes and configurations of a transistor module 31 and the external lead terminal 32. For instance, as shown in FIG. 7, in a case where an internal terminal portion 37 is also provided at a position distant from the external terminal portion 34 via an intermediate terminal portion 36 having a bent portion 36a, it is necessary to provide slits 36b between the bent portion 36a and the external terminal portion 34 and provide a slit 36c between the bent portion 36a and the internal terminal portion 37. However, to reduce the transient voltage .DELTA.V.sub.1 sufficiently, it is necessary to form a multiplicity of grooves 36a and 36b, so that it is essential that the width of the intermediate terminal portion 36 be wide. In addition, an increase in the number of the slits 35a, 36b, and 36c formed in the external lead terminal 32 results in an increase in the number of processing operations, and the problem of the increased manufacturing cost of the transistor module 21 also results.